1. Technical Field
The invention disclosed broadly relates to arbitration on a communications bus and more particularly relates to a method and apparatus for performing multilevel bus arbitration.
2. Background Art
Computers are often connected Via a network to form a local area network (LAN) or a wide area network (WAN). Using this network, computers and peripheral devices are able to communicate with each other. Only one device can be given access to the network at any given time. The Micro Channel (TM IBM Corporation) architecture or MCA is an IBM proprietary backplane communications protocol used in the PS/2 and RS6000 family of computers to allow I/O adapters to communicate with a host processor or other I/O adapters. The arbitration mechanism used in the Micro Channel architecture allows up to 16 adapters to compete for the bus at any given time.
Several Micro Channel interface chips have been developed within IBM and by others to allow communication on the Micro Channel architecture bus. The control chips provide the necessary control and interface logic to allow an I/O adapter to participate in MCA data transactions.
As part of the MCA protocol, an I/O adapter must arbitrate for the Micro Channel bus. The arbitration mechanism implemented in the Micro Channel architecture assigns only a single arbitration value to an adapter at any given time which it uses to compete for the bus. If an adapter wishes to have its arbitration level changed, to either increase or decrease its arbitration priority, the adapter must interrupt the host processor to request the change upon which the host processor reassigns the arbitration level to the adapter. For adapters which execute both high and low priority applications, a significant performance penalty is encountered by not being able to select between multiple arbitration levels when arbitrating for the Micro Channel bus.
Even though there are numerous MCA interface chips which have been developed, both within and outside of the IBM Corporation, to support the Micro Channel protocol, none of them allows selection between arbitration levels when competing for a communications bus.